Generally, in technology an accelerating tendency for miniaturizing can be observed and the thereby evolving fields of micro- and nanotechnology involve a demand for instruments that allow to manufacture such tiny systems and/or to inspect these. Especially the components and structures in the field of semiconductor technology, becoming smaller and more complex, involve particle-optical systems for inspecting and manufacturing the sub-micrometer sized functional components, wherein these particle-optical systems ought to provide a sufficiently high resolution. For focussing the charged particles, magnetic lenses, alone or in combination with electrostatic lenses, are conventionally used in such particle-optical systems. The achieved resolution in such a system substantially depends on the imaging properties of the system as a whole.
A known embodiment of a magnetic lens is referred to as “single pole” lens or also “Snorkel” lens and generally includes a single, usually conical pole piece which, exited by a coil, generates a magnetic field in the region of an object to be inspected.
Magnetic lenses with so-called axial pole piece gap are known as well. Such lenses usually include two pole pieces extending in a direction towards an object plane of the objective lens. The inner diameters of the pole pieces are of equal size at their ends disposed closest to the object. Thereby, an axial gap is formed arranged substantially parallel to an optical axis of the magnetic lens and the ends of the pole pieces are arranged at the same distance from the optical axis. Compared to the single pole (“Snorkel”) lens, for which the generated magnetic field extends a far amount into the space located in front of the objective lens and thus includes the object, the magnetic lens with an axial pole piece gap has a characteristic steep decrease of the magnetic field.
Combination lenses formed of a magnetic lens and an electrostatic lens are known. In these types of lenses, the electrostatic lens may for example be arranged in form of an immersion lens inside the magnetic lens, such that electrostatic field and magnetic field overlap.